1. Field of the Invention
This invention covers an underground storage tank for storage of liquids of any nature whatsoever.
2. Description of the Related Art
These tanks are usually cylindrical with horizontal axis and are consisting of an internal the metal containment structure, an external static facing in touch with the soil in plain simple or reinforced concrete and a filler located between the metal containment structure and the static facing.
Large underground storage tanks are normally built with hot press rolled metal elements or large sized rings, the transverse edges of which are provided with angle sections, apt automatically to create transverse and longitudinal channels after the rings have been jointed and with the aid of flat sections, for easy detection and sealing of any leaks.
Such transverse channels can also be obtained with omega shapes and flat sections, welded onto the rings which, in this case, are without angle-shapes. The latter solution is preferable since hot-press rolling of the rings, stiffened by shaping, is rather difficult.
These large sized metal elements allow for an easier and lower-cost assembly of the metal structure erected at the crown of the static concrete facing on which it is resting and which externally protects it, since handling and installation of these elements is much easier and much more rational than achievable with previous assembly and erection techniques.
The metal containment structure is supported by the static reinforced or plain concrete facing, by means of U-bolts and a filler is poured or injected into the hollow space between the metal structure and static facing.
The static stability of these large underground tanks is currently guaranteed by providing the metal structure and the static facing with a separate static stability without taking their reciprocal interaction into account. An important advantage in building such large sized underground tanks can be achieved if the system is conceived as a metal containment structure, static facing, filler and surrounding soil, i.e. as one single interacting complex structure so that all its components are all together and directly contributing to the structural stability of these tanks which will thus be configurated according to the underground cavity-walled storage tank design.
By meeting this requirement, the resistance parameters and thickness of the metal structure and of the static facing may be reduced by reducing the quantity of material.
These tanks, thus designed, are known from EP-A-0325683 and its related U.S. Pat. No. 4,915,545, in which a special hot blown bitumen is injected between the static facing and the metal shell at a temperature of about 200.degree.--200.degree. C.
The construction of such underground storage tanks, according to these known patents is therefore rather inexpensive and stress resisting, including seismic stresses, but this invention has the aim to provide an even better structural stability to the three tank components, i.e. its static facing, metal shell and hot blown bitumen filler.
The metal shell is indeed the tank component most exposed to stresses and strains, especially during filling and emptying of the tank. Such strains in the metal shell will generate deformation of the filler, although these strains are usually well absorbed since hot blown bitumen is used as a filler, having excellent viscous-elastic properties.